Open and closed loop calibration methods are generally known and applied in a variety of contexts. One such context is in the calibration of Hall-effect magnetic field sensors. Hall-effect magnetic field sensors are solid state magnetic sensor devices that can be used to measure magnetic fields. Applications of Hall-effect magnetic field sensors require high accuracy; however, they are known to suffer from variation and drift in sensitivity with process variations, temperature, and package stress changes. The conventional solution to control for the complex temperature dependence that Hall-effect sensors exhibit is to implement so-called “open-loop” temperature compensation circuitry configurations. Fine-tuning (or “trimming”) the sensitivity of each part for the process variation may be carried out, and the changes in sensitivity with temperature and stress may be compensated for by using on-chip temperature and stress sensors and pre-evaluated compensation tables. This approach requires expensive multi-point characterization of individual devices and re-calibration over time. The magnetic field excitation for calibration of the sensor can be created using an on-chip current coil or external magnetic field sources. Calibration, however, can only be performed when the device is offline and hence, not in operation, as the signal to be measured can interfere with the calibration signal.
As an alternative to the open-loop scheme, closed-loop methods have been implemented to perform continuous calibration in the absence of external magnetic fields. Closed-loop calibration typically works as follows: a known magnetic field is applied to the device (a method of generating known magnetic field would be: a known temperature-insensitive current is passed through an on-chip/off-chip coil/other suitable trace near the sensor), the sensor output is then compared with the desired response, and the sensor sensitivity/gain is adjusted to minimize the comparator error. This results in much higher accuracy than the open-loop configuration.
A known issue with conventional closed-loop calibration of a Hall-effect sensor is that the calibration current near the Hall-effect sensor can generate enough heat that it changes the operating temperature, resulting in a change of sensitivity and affecting the primary measurement. Additionally, closed-loop calibrations have been demonstrated to perform well in the absence of external magnetic fields, but completely eliminating interference in real-world applications is non-trivial and can require offline calibration in a magnetically shielded environment.